Apache Workbench ab is a commandline tool used to measure website performance.

The best example I have seen is benchmarking a website with caching turned on and then again with caching turned off to determine the effectiveness of the a caching strategy.

The example below sends 1000 web requests to kb.sahwira,net

$ ab -n 1000 -c 10 https://kb.sahwira.net/

This is ApacheBench, Version 2.3 <$Revision: 1843412 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking kb.sahwira.net (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Completed 600 requests
Completed 700 requests
Completed 800 requests
Completed 900 requests
Completed 1000 requests
Finished 1000 requests

Server Software:        Apache
Server Hostname:        kb.sahwira.net
Server Port:            443
SSL/TLS Protocol:       TLSv1.2,ECDHE-RSA-CHACHA20-POLY1305,2048,256
Server Temp Key:        X25519 253 bits
TLS Server Name:        kb.sahwira.net

Document Path:          /
Document Length:        15767 bytes

Concurrency Level:      10
Time taken for tests:   26.587 seconds
Complete requests:      1000
Failed requests:        0
Total transferred:      16185000 bytes
HTML transferred:       15767000 bytes
Requests per second:    37.61 [#/sec] (mean)
Time per request:       265.872 [ms] (mean)
Time per request:       26.587 [ms] (mean, across all concurrent requests)
Transfer rate:          594.49 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:       15  115  65.6     98     469
Processing:    38  149  65.3    138     576
Waiting:       24   90  41.2     81     267
Total:         69  264  98.1    240     721

Percentage of the requests served within a certain time (ms)
  50%    240
  66%    270
  75%    299
  80%    327
  90%    382
  95%    472
  98%    546
  99%    624
 100%    721 (longest request)